Compressed media may include compressed audio, compressed video, or other forms of media.
One popular compressed media is the so-called MP3 music format. Conventional MP3 format allows a variable audio quality depending on the number of bits that are assigned to carry the audio. For example, a smaller bit rate produces a more coarse approximation of the final signal. This provides a smaller file size as a tradeoff for a lower quality audio playback. A larger bit rate provides a larger file, but more closely approximates the way in which the actual signal is played. Variable bit rates may change the bit rate from area to area.
The sample rate of MP3s may also change. A standard MP3 sample may be 44.1 kHz, the same sampling rate is used in a conventional CD. In addition, however, other sampling rates can exist. People conventionally use very low sampling rate, such as 8 kilohertz for simple spoken audio. On the other hand, professional music may be sampled at 96 kHz or more.
In general, a smaller file means less accurate playback; a larger file means more accurate playback.
The MP3 file uses a header with various parameters about the file part. The header is followed by a data area. The header and data area together is called a frame. Each frame may include a specified amount of data, and may have data for any desired amount of time. In a typical MP3 file, the frame typically might around or somewhat less than a second.
Techniques of restricting play and/or transfer of media signals are known. Digital rights management on the MP3s have been met with significant resistance.